The present invention relates to polymer electrolyte fuel cells for use in portable power sources, electric vehicle power sources, domestic cogeneration systems, or the like.
A fuel cell including a polymer electrolyte membrane generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen, such as air. The fuel cell includes a polymer electrolyte membrane that selectively transports hydrogen ions and a pair of electrodes (anode and cathode) formed on both sides of the polymer electrolyte membrane. The electrode includes a catalyst layer formed on each side of the polymer electrolyte membrane and a gas diffusion layer formed on the outer face of the catalyst layer. The catalyst layer is composed mainly of a carbon powder carrying a platinum group metal catalyst, and the gas diffusion layer has excellent gas permeability and electronic conductivity.
Around the electrodes, gas sealing materials or gaskets are arranged so as to sandwich the polymer electrolyte membrane, in order to prevent the supplied fuel gas and oxidant gas from leaking out and mixing together. The gas sealing materials or gaskets are combined integrally with the electrodes and the polymer electrolyte membrane beforehand. This combined structure is called an electrolyte membrane-electrode assembly (MEA).
Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and connecting adjacent MEAs electrically in series. The separator plates have a gas flow channel for supplying a reactive gas to the electrode surface and discharging generated gas and surplus gas. Although the gas flow channel may be provided independently of the separator plate, grooves are usually formed in the surface of the separator plate to provide the gas flow channel.
In order to supply the reactive gas to the grooves, it is necessary to use a piping jig which, depending on the number of separator plates, branches off from the supply pipe of the reactive gas into the grooves of the respective separator plates. This jig is called a “manifold,” and the above-described type of manifold, connecting the supply pipe of the reactive gas with the grooves of the respective separator plates, is called an “external manifold.” A manifold having a simpler structure is called an “internal manifold.” The internal manifold includes through-holes that are formed in the respective separator plates with the gas flow channel. The throughholes are connected to the inlet and outlet of the gas flow channel so that the reactive gas is supplied directly from these holes to the gas flow channel.
Since the fuel cell generates heat during operation, it is necessary to cool the fuel cell with cooling water or the like, in order to keep the cell under good temperature conditions. A cooling section for flowing cooling water therein is usually provided for every group of one to three cells. The cooling section is often provided by forming a cooling water flow channel in the backside of the separator plate. The MEAs and the separator plates are alternately stacked to form a stack of 10 to 200 cells, and a current collector plate and an insulator plate are attached to each end of the cell stack. The resultant stack is sandwiched between end plates and clamped with clamping bolts from both ends. This is the structure of a typical fuel cell.
The gaskets, used in such a polymer electrolyte fuel cell, seal in gas while allowing the separator plate to contact the electrode, so the gaskets are required to have high dimensional accuracy, sufficient elasticity, and sufficient interference. To meet such requirements, sheet-like gaskets made of resin or rubber and O-rings made of rubber have been used in the related art.
Also, attempts have recently been made to decrease the load applied to the gaskets, in order to reduce the clamping load of the stack and achieve weight reduction, simplification and cost reduction of the constituent members. One such attempt is to employ gaskets having a triangular or semicircular cross section in addition to the O-ring gaskets (e.g., Japanese Laid-Open Patent Publication No. 2002-141082). Other attempts to mount the gaskets on the separator plates, not the MEAs, have also been made for ease of assembly (e.g., Japanese Laid-Open Patent Publication No. 2002-231264).
FIG. 13 is a longitudinal sectional view of the vicinity of O-ring gaskets of a related art fuel cell. O-rings 236 and 246 are fitted into O-ring grooves 236a and 246a formed in an anode-side separator plate 210 and a cathode-side separator plate 220. The O-ring 236 presses an electrolyte membrane 231 against the cathode-side separator plate 220, and the O-ring 246 presses the electrolyte membrane 231 against the anode-side separator plate 210. As a result, the gaps between the electrolyte membrane and these anode-side and cathode-side separator plates are sealed by the O-rings 236 and 246. Reference character 232a represents an anode, 232b a cathode, 212b a fuel gas flow channel, and 223b an oxidant gas flow channel.
In this way, since the sealing is performed at two locations by the O-ring gaskets, this related art sealing method has a problem of requiring a large sealing space.
Also, with internal-manifold-type separator plates, the gas sealing section extends from the manifold to the electrode section, so the electrolyte membrane needs to be large enough to cover the manifold, which leads to high costs. Further, as the membrane size increases, the handling of the membrane becomes more difficult during the assembly, because the membrane has a thickness of approximately 25 to 50 μm.
On the other hand, when the membrane size is decreased to reduce costs and improve handling and strength, the electrolyte membrane is covered with a somewhat rigid protective film. If a unit cell includes such a downsized electrolyte membrane, the thickness of the electrolyte membrane creates a difference in height around the electrodes, thereby posing a problem of degradation in sealing characteristics. Further, when the above-mentioned O-rings are used as the gaskets, it takes considerable time to fit the O-rings, which are thin and not rigid themselves, without twisting them during the assembly of the cell stack, thereby presenting a problem of high manufacturing costs.
Also, since the separator plates are made of a conductive material, inclusion of conductive foreign matter in an assembling process may lead to short-circuits between the separator plates sandwiching the MEA. Warpage or distortion of the separator plates, distortion resulting from the assembly of a fuel cell, or the like may also lead to short-circuits between the separator plates. Further, inclusion of conductive foreign matter between the assembled fuel cell and a heat insulator may lead to short-circuits.
Moreover, when the O-rings are used for sealing, a fuel cell is assembled by stacking the components of the fuel cell, for example, placing an MEA on a separator plate, and then, placing thereon another separator, or a gasket and another separator. In this assembling process, the gasket or the separator plate to be placed on the MEA is assembled along the guide pins of an assembling jig. However, since these components have dimensional errors, they cannot be assembled efficiently and successfully, unless the clearance between the gasket and the MEA is large enough to accommodate the dimensional errors. Thus, reactive gas passes through the resultant clearance between the gasket and the electrode, bypassing the gas flow channel of the separator plate.
Due to assembling errors of the MEA and the gasket and other factors, the size of the clearance between the electrode and the gasket varies from cell to cell, resulting in variations in pressure losses in the respective cells. The varied pressure losses in the respective cells cause variations in the flow rate of reactive gas, because the reactive gas flows into the respective cells of a fuel cell in amounts depending on the varied pressure losses in the respective cells. As a result, the respective cells exhibit performance variations, which involve such detrimental effects as deterioration in generated voltages, durability and low-output-operation stability, etc. These detrimental effects are remarkable on the anode side, where the reactive gas utilization is comparatively high.
Also, reducing the clearance between the gasket and the electrode requires an improvement in the accuracy of dimensions of the components, thereby inviting a decrease in yield and an increase in component costs. Further, when the clearance is reduced, the assembly of the components becomes difficult, and the assembling reliability lowers. Thus, sealing failure is caused, for example, by part of the electrode overlapping the sealing section, and a tensile stress and a shearing stress are excessively applied to the electrolyte membrane, inducing breakage of the electrolyte membrane, a decline in durability, and the like.
Recently, it has been requested to heighten the degree of humidification of the supplied gases, in order to enhance cell performance. On the oxidant gas side, water is generated by the reaction, and the water should be discharged from the electrode promptly and stably. If the clearance between the gasket and the electrode is reduced, or substantially removed, to prevent the above-described bypass of reactive gas while using the related art gasket, a large pressure is necessary for discharging the water from the electrode, inviting a problem of system efficiency degradation. Moreover, decreasing the clamping force of the sealing section is also a problem to be solved, for the purposes of weight reduction, size reduction and cost reduction of clamping members of the stack. Under such situations, there is a demand for simple and effective sealing configurations.